Fluorine-containing polymers are high molecular weight materials having excellent heat resistance, solvent resistance, weather resistance, chemical resistance, etc., and in recent years have been used for various applications making the best use of such characteristics.
As a method of preparing fluorine-containing polymers, there are known methods such as a solution polymerization method, suspension polymerization method and emulsion polymerization method. With respect to a polymerization medium for the solution polymerization and suspension polymerization, inactive solvents such as chlorofluorocarbon (CFC) and hydrochlorofluorocarbon (HCFC) have been usually used from the point of providing a high molecular weight copolymer and from the viewpoint of a polymerization rate. Examples of CFC are CCl2FCClF2 (CFC113), CClF2CClF2 (CFC114) and the like, and examples of HCFC are CH3CCl2F (HCFC141b), CH3CCl2F (HCFC142b) and the like. From the viewpoint of easiness of handling, CFC have been mainly used.
However depletion of an ozone layer was taken up as an international issue of environmental disruption in a global scale and use of specific CFC and HCFC which are substances causing this issue was prohibited. Therefore development of alternatives which are used instead of CFC and HCFC for preparing fluorine-containing polymers is greatly demanded.
As the alternatives to CFC, there have been proposed hydrofluorocarbons (HFC) having hydrogen atom and fluorocarbons (FC) since they have a low ozone depletion potential. However when a solvent not containing chlorine was used as a polymerization solvent, there were problems with lowering of a polymerization rate and in addition, lowering of appearance and mechanical properties of a coating film formed using the obtained fluorine-containing polymer.
Also WO9948937 discloses a technology of polymerizing a vinylidene fluoride copolymer in a HFC solvent, but there is neither suggestion as to polymerizability of copolymers other than the vinylidene fluoride copolymer nor description that a fluorine-containing solvent having an ability of dissolving a specific monomer is advantageous in a combination of fluoroolefins other than vinylidene fluoride from the viewpoint of polymerization efficiency and applied characteristics.
Under such a background as mentioned above, an object of the present invention is to provide a process for efficiently preparing a fluorine-containing polymer without using CFC and HCFC having a high polymerization rate and a high ozone depletion potential, in which a coating film formed using the obtained fluorine-containing polymer has excellent mechanical properties, solvent resistance and chemical resistance.